1. Field of the Invention
The present invention relates to a walking assist device for assisting a user in walking.
2. Description of the Related Art
Conventionally, as a walking assist device, there has been known one provided with a load transmit portion, a foot mounting portion mounted to a user's foot, and a leg link disposed between the load transmit portion and the foot mounting portion (for example, refer to Patent Document 1: Japanese Patent Laid-Open No. 2007-20909). The walking assist device is configured to transmit a force generated from the leg link to the user's trunk via the load transmit portion.
The leg link in the walking assist device includes an upper first link portion connected to the load transmit portion via a first joint portion, a lower second link portion connected to the foot mounting portion via a second joint portion, a middle third joint portion connected to the first link portion and the second link portion in such a way that the first link portion and the second link portion can stretch and bend freely, and a drive mechanism to drive the third joint portion.
Thereby, the load applied to a leg of the user can be alleviated according to the force generated by the drive mechanism in the direction of decreasing a flexion angle of the third joint portion (same as the direction of stretching the leg link).
In the device disclosed in Patent Document 1, the load transmit portion is composed of a seat member on which the user sits astride, and the first joint portion is composed of an arc-shaped guide rail which is connected to the seat member and is longitudinal in an anteroposterior direction with the center of curvature located above the seat member and a slider which is fixed at an upper end portion of the first link portion and is movably engaged in the guide rail.
Thereby, the center of curvature of the guide rail is equivalent to the swing fulcrum for the leg link of the first joint portion in the anteroposterior direction. Since the swing fulcrum is located above the seat member, the seat member can be prevented from inclining greatly in the vertical direction due to the shifting in the weight of the user.
Further, in the device disclosed in Patent Document 1, the slider is engaged to a part of the guide rail which is positioned at a rear side to the connection line connecting the center of curvature of the guide rail and the joint shaft of the third joint portion. Thereby, the swing stroke of the leg link to the forward so as to follow the forward movement of a free leg (the leg with foot leaving away from the floor) of the user can be assured without increasing the length of the guide rail to the forward direction too much; consequently, it is expected to reduce the size of the first joint portion.
The drive mechanism described in an embodiment of the Patent Document 1 is provided with a rotary actuator mounted to the first link portion, and a wire-typed force transmit portion configured to transmit a force from the rotary actuator to the third joint portion via a wire. However, the drive mechanism is not limited thereto, specifically, it is acceptable that the drive mechanism is provided with the rotary actuator mounted to the first link portion, a drive crank arm disposed on an output shaft of the rotary actuator, a driven crank arm fixed to at the second link portion concentrically to a joint shaft of the third joint portion, and a connection link with one end pivoted at the drive crank arm and the other end pivoted at the driven crank arm.
Generally, it has been considered to configure the drive mechanism as a parallel link mechanism by disposing the connection link in such a way that a connection line connecting a pivot portion of the connection link at which the drive crank arm is pivotally mounted and a pivot portion of the connection link at which the driven crank arm is pivotally mounted is parallel to a connection line connecting the output shaft of the rotary actuator and the joint shaft of the third joint portion.
However, if the inertial moment of the leg link around the first joint portion is greater, when the user swings the free leg to the forward, the load applied to the free leg due to the inertial moment of the leg link will become greater. Therefore, it is desired to reduce the inertial moment of the leg link. In this regarding, if the rotary actuator mounted at the first link portion is made lighter, the inertial moment of the leg link can be reduced. However, in order to generate the desired assist force for the leg link, it is necessary for the rotary actuator to output a torque of at least a certain magnitude; therefore, there is a limit on reducing the weight of the rotary actuator.
To solve this problem, it has been considered to increase the length of the driven crank arm longer than the length of the drive crank arm to decrease the rotational angular velocity of the driven crank arm slower than the rotational angular velocity of the drive crank arm so as to increase the torque transmitted to the driven crank arm, in other words, to increase the drive torque of the third joint portion greater than the output torque of the rotary actuator. However, this solution brings about the following problem, that is, for the leg link with the first link portion and the second link portion connected by the third joint portion in such a way that the first link portion and the second link portion can stretch and bend freely, the telescopic velocity of the leg link obtained by differentiating the length of the leg link (the length of a line segment connecting the first joint portion at the upper end and the second joint portion at the lower end) by the flexion angle of the third joint portion slows down as the flexion angle of the third joint portion decreases. Therefore, in order to improve the controllability in a small range of the flexion angles of the third joint portion, it is necessary to make the flexion angle vary faster. Accordingly, in the device where the rotational angular velocity of the driven crank arm is slower than the rotational angular velocity of the drive crank arm, the required rotational velocity of the rotary actuator would be greater, which makes it difficult to reduce the weight of the rotary actuator.